Rapid admission has long been a core question in the development of absorbing articles. The rapid admission of liquid into an absorbing article is necessary in order for the liquid to be capable of being dealt with by the absorbing structure. This is especially true in the case of repeated wettings.
In the case of repeated wettings, the storage layers often become saturated locally. This impairs the function of the surface layer and the distribution layers, if any, the purpose of which is rapidly to admit and distribute the liquid. The risk of leakage taking place is increased if admission takes place too slowly. The absorbing article can also become uncomfortable to wear when the surface that faces towards the wearer is wetter for a longer period than normal.
A number of different procedures for improving admission in an absorbing article have been developed over the years. Perforated surface layers, hydrophilic wet areas on the surface layers, distribution layers with a high pore volume (also known as the void volume), to mention only a few. The need remains, however, for further improvements in this area.
There is also a need for solutions which take account of feces handling. Feces handling in absorbing articles and their structures is very often totally neglected when new materials and concepts are being developed. This is in spite of the fact that feces handling remains one of the major unresolved problems in today's diaper industry. Feces handling calls for relatively large open passages in the surface layer to enable the feces to penetrate down into an absorption structure. It is not advantageous, however, for the surface layers to exhibit conspicuous holes. This is because the wearer may draw premature conclusions about the function of the article and its resistance to leakage.
The use of a material that is deformed when it becomes wet is previously disclosed. The expression deformation is used here to denote a change in the structure. A change of this kind may involve the expansion or shrinkage of the material. However, the material must preferably not undergo any chemical change. The deformation is more often than not initiated by a special factor, for example temperature, pH or in the event of contact with liquid. A layer which meets these criteria can be termed a deformation layer.
Described in U.S. Pat. No. 5,885,264 is an absorbing article with a layer positioned on the surface layer, which shrinks when it becomes wet. The purpose of this layer is to indicate that the wearer has wet the diaper, and to ensure that the wearer experiences a sensation of wetness against the skin. When the wearer is an infant, a product of this kind can be used, for example, for potty training.
Described in EP 846,454 A1 is a backing layer which shrinks when it is exposed to liquid. The material that is used to impart the shrinking property contains polymers such as polyvinyl alcohol with a degree of hydrolysis of at least 88%, preferably 95%, and if at all possible at least 98%. The polyvinyl alcohol is produced by the hydrolysis of polyvinyl acetate. The expression degree of hydrolysis is used here to denote the percentage expressed in mol by which the acetate groups have been replaced with hydroxyl groups. The higher the degree of hydrolysis of the polyvinyl alcohol, the higher is the crystalline nature of the polyvinyl alcohol. This is explained in EP 846,454 A1 by the fact that the crystallinity of the polyvinyl alcohol is in direct proportion to the ability of the polymer to shrink when it comes into contact with liquid. A high crystallinity thus gives greater shrinkage, i.e. the material shrinks more.
The crystallinity can also be improved, and with it the polymer's property of being able to shrink, by increasing the degree of replaced acetate groups by purely chemical means. The crystallinity can also be improved by processing the polymer by adequate mechanical stretching. This is customarily achieved, for example, during the formation of fibers and structures such as layers and films.
When polyvinyl alcohol shrinks, this is attributable to the partial decomposition and/or plasticization of the crystalline structure of the polyvinyl alcohol through contact with the liquid. This takes place at a molecular level. Due to the fact that the crystalline structure was formed during stretching, any residual unbalanced stress remains in the material. The crystals are plasticized on contact with liquid and become softer. As a result of this, the internal stresses in the material become lower and permit the material to resume its earlier structure.
Described in U.S. Pat. No. 6,605,349 B2 is a deformable shrink fiber. The dimensions of the fiber are said to be stable at normal body temperature (ca 37° C.) in its dry state. The dimensions are unstable, however, in the wet state at the same temperature, i.e. the fiber shrinks and is deformed. The fiber contains a fiber-shaped polymer that has a dry glass transition temperature (“dry Tg”) greater than or equal to 42° C., and a wet glass transition temperature (“wet Tg”) less than or equal to 32° C.
Further examples of shrink fibers are described in U.S. Pat. No. 4,357,938, where shrink fibers are laid in a longitudinal direction alongside the absorption body in order to cause it to flex when it becomes wet.
Room is accordingly available for a solution which improves the currently available absorbing structures and absorbing articles, on the one hand a solution which permits more rapid admission in conjunction with repeated wetting, and on the other hand a solution which can provide feces with the possibility of penetrating down into the absorbing structure and, in so doing, avoid irritation to a wearer's skin.